In the field of missile guidance systems, previous autopilots are designed to function properly when the torque and the translational forces on the craft are constants. During the flight of a missile or other craft, aerodynamic forces will be present. As long as the environmental parameters and direction of flight remain unchanged the forces will be nominally constant. If the autopilot, however, operates to change the direction of flight, or the environmental parameters change, such as by varying the crafts altitude, the aerodynamic forces will be altered.
If these forces vary in a predicable manner or to a known degree, additional circuity may be including within the autopilot to compensate for these variations. For example, the variations of the aerodynamic forces caused by the crafts change of altitude are compensated for in at least one prior autopilot by a switching circuit to automatically vary the autopilot's electronic gain in response to the change in altitude.
If the autopilot, however, operates to change the direction of flight, or the environmental parameters change, in an unpredictable or unknown fashion, the alteration causes M and N to be unpredictable and unknown variables, wherein M is an unknown function of the torque on the craft, and N is an unknown function of the translational force on the craft. In such a case, M and N are extremely difficult to measure, and therefore, cannot be compensated for by employing a switching system to vary the gain, or by employing any of the other presently used compensation systems. As a result, in these situations, or when the forces inherently applied to the craft during its flight approach zero, all previous autopilots saturate or otherwise malfunction.
M is defined as the pitch acceleration coefficient in rad/sec.sup.2, and is defined by EQU M=(1/J)(C.sub.N qsL.alpha.+TL.sub.t .delta.)=.theta.
wherein:
J=Polar moment of inertia in pitch (slug-ft.sup.2) PA1 C.sub.N.sbsb..alpha. =f(M).f(.alpha.)=Linearized body force coefficient (per degree) PA1 q=1/2 ev.sup.2 =Aerodynamic pressure (lb/ft.sup.2) PA1 s=Missile reference area (ft.sup.2) PA1 L=f(t)+f(M).f(.alpha.)=Distance between C.P. and C.G. (ft) PA1 T=f(t)=Rocket motor total thrust (lb) PA1 L.sub.t =f(t)=Distance between tail force application point and C.G. (ft). PA1 .theta.=pitch angular acceleration (rad/sec.sup.2) PA1 .alpha.=angle of attack (degrees) PA1 .delta.=nozzle deflection (degrees) PA1 N is the missile cross-body acceleration coefficient in ft/sec.sup.2, and is defined by EQU N=(1/m)(C.sub.N.alpha. qs.alpha.+T.delta.-C.sub.X cos.sup.2 .alpha.qs)=y PA1 wherein:
m=f(t)=Missile mass (slugs) PA2 C.sub.N.alpha. =Linearized aerodynamic body normal force coefficient (per degree) PA2 q=1/2 pv.sup.2 =Aerodynamic pressure (lb/ft.sup.2) PA2 s=Missile reference area (ft.sup.2) PA2 T=f(t)=Rocket motor total thrust (lb) PA2 C.sub.x =f(M).f(.alpha.)=Aerodynamic axial force coefficient PA2 .alpha.=missile angle of attack (degrees) PA2 y=translational acceleration (ft/sec.sup.2) PA2 .delta.=nozzle deflection (degrees)
The present invention is an adaptive autopilot wherein the value of the variable parameters M and N are identified by a parameter identification circuit and continuously updated throughout the flight, although they are not directly measured. The identified variable parameters are then compensated for by the present invention. As a result, flight stability is maintained even when the craft performs extraordinary gymnastics.